My Hyundai Sonata 2008 smells sour, when the air ventilation is set to external rather than internal.
I always immediately turn it to internal air to avoid the smell.
Where is the ventilation hole/filter and where do I need to clean?
Your Sonata has a cabin air filter (CAF). You should try to change that out first. More than likely, you have some mildew built up somewhere. The CAF is the first place to start.